Aperture controlled laser beam shaping techniques for scanning optical code

ABSTRACT

An improved optical system for scanning optical codes is disclosed. A laser beam produced thereby is a diverging, nearly diffraction free beam and may be used to produce an elongated scanning spot. An aperture may be employed such that variation in size of the aperture in one orthogonal dimension varies the size of the scanning spot in another orthogonal dimension.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/714,511 filed Nov. 17, 2000 to Gurevich et al. entitled “ImprovedBeam Shaping System and Diverging Laser Beam For Scanning Optical Code”,now pending which is hereby incorporated by reference herein.

This application is also a continuation-in-part of U.S. application Ser.No. 09/867,399 filed May 31, 2001 to Bergstein et al. entitled “BeamShaping Optical Scanners”, now pending which is hereby incorporated byreference herein. The Bergstein application is a continuation-in-part ofU.S. application Ser. No. 09/330,053 filed Jun. 11, 1999 to Bergstein etal. entitled “Beam Shaping For Optical Scanners.”, now abandoned.

FIELD OF THE INVENTION

This invention relates to optical scanning devices such as bar codescanners, and more particularly to beam-shaping systems for generating ascanning laser beam adapted for use with a selected broad range ofworking distances and symbol densities.

BACKGROUND AND OBJECTS

Bar code readers are known in the prior art for reading varioussymbologies such as UPC bar code symbols appearing on a label or on thesurfaces of an article. The bar code symbol itself is a coded pattern ofindicia comprised of a series of bars of various widths spaced apartfrom one another to bound spaces of various widths, the bars and spaceshaving different light reflecting characteristics. The readers inscanning systems electro-optically transform the graphic indicia intoelectrical signals, which are decoded into information, typicallydescriptive of the article or some characteristic thereof. Suchinformation is conventionally represented in digital form and used as aninput to a data processing system for applications in point-of-saleprocessing, inventory control and the like. Scanning systems of thisgeneral type have been disclosed, for example, in U.S. Pat. No.5,600,121, assigned to the same assignee as the instant application.Such systems may employ a portable laser scanning device held by a user,which is configured to allow the user to aim the device, and moreparticularly, a scanning laser light beam, at a targeted symbol to beread.

The light source in a laser scanner bar code reader is typically asemiconductor laser. The use of semiconductor devices as the lightsource is especially desirable because of their small size, low cost andlow voltage requirements. The laser beam is optically modified typicallyby an optical assembly, to form a beam spot of a certain size at thetarget distance. It is preferred that the cross section of the beam spotat the target distance be approximately the same as the minimum widthbetween regions of different light reflectivity, i.e., the bars andspaces of the symbol.

In the laser beam scanning systems known in the art, the laser lightbeam is directed by a lens or other optical components along the lightpath toward a target that includes a bar code symbol on the surface. Themoving-beam scanner operates by repetitively scanning the light beam ina line, pattern or series of lines across the symbol by means of motionof a scanning component, such as a moving mirror placed in the path ofthe light beam. The scanning component may either sweep the beam spotacross the symbol and trace a scan line across the pattern of thesymbol, or scan the field of view of the scanner, or both.

Bar code reading systems also include a sensor or photo detector whichdetects light reflected or scattered from the symbol. The photo detectoror sensor is positioned in the scanner in an optical path so that it hasa field of view which ensures the capture of a portion of the lightwhich is reflected or scattered off the symbol. The light is detectedand converted into an electrical signal.

Some bar code reading systems are “retro-reflective”. In aretro-reflective system, a moving optical element such as a mirror isused to transmit the outgoing beam and receive reflected light.Non-retro-reflective systems typically employ a moving mirror totransmit the outgoing beam and a separate detection system with a wide,static field of view.

Electronic circuitry and software decode the electrical signal in adigital representation of the data represented by the symbol that hasbeen scanned. For example, the analog electrical signal generated by thephoto detector is converted by a digitizer into a pulse or modulateddigitized signal, with the widths corresponding to the physical widthsof the bars and spaces. Such a digitized signal is then decoded, basedon the specific symbology used by the symbol, into a binaryrepresentation of the data encoded in the symbol, and subsequently tothe information or alphanumeric characters so represented. Such signalprocessors are disclosed in U.S. Pat. No. 5,734,152 assigned to SymbolTechnologies, Inc. which patent is hereby incorporated by reference.

Different bar codes have different information densities and contain adifferent number of elements in a given area representing differentamounts of encoded data. The denser the code, the smaller the elementsand spacings. Printing of the denser symbols on an appropriate medium isexacting and thus is more expensive than printing symbols with largerelements. The density of a bar code symbol can be expressed in terms ofthe minimum bar/space width, also called “module size”, or as a “spatialfrequency” of the code, which is in the inverse of twice the bar/spacewidth.

A bar code reader typically will have a specified resolution, oftenexpressed by the module size that is detectable by its effective sensingspot. For optical scanners, for example, the beam spot size may besomewhat larger than the minimum width between regions of differentlight reflectivities, i.e., the bars and spaces of the symbol. Theresolution of the reader is established by parameters of the beam sourceor the detector, by lenses or apertures associated with either the beamsource or the detector, by the angle of beam inclination with respect tothe plane of the symbol, by the threshold level of the digitizer, by theprogramming in the decoder, or by a combination of two or more of theseelements. The photo detector will effectively average the lightscattered from the area of the projected spot which reaches the detectoraperture.

The region within which the bar code scanner is able to decode a barcode is called the effective working range of the scanner. Within thisrange, the spot size is such as to produce accurate readings of barcodes for a given bar code density. The working range is dependent onthe focal characteristics of the scanner components and on the modulesize of the bar code.

Many known scanning systems collimate or focus the laser beam using alens system to create a beam spot of a given diameter at a prescribeddistance. The intensity of the laser beam at this point, in a planenormal to the beam (ideally approximately parallel to the scannedsymbol), is ordinarily characterized by a “Gaussian” distribution with ahigh central peak. Gaussian beams typically have a profile along theiraxis of propagation exhibiting a waist (collimated) zone with limiteddivergence followed by a divergence zone thereafter. The collimated zonedetermines a depth of field (focusing range) for maximum bar codedensity. The working range is defined as the region within which thescanned beam spot is sufficiently well formed that its detectedscattered radiation can be decoded by the scanner. But as the distancebetween the scanner and the symbol moves out of the working range of thescanner, which is typically only a few inches in length, the Gaussiandistribution of the beam spot greatly widens, preventing accuratereading of a bar code. Such scanning systems, accordingly, must bepositioned within a relatively narrow range of distances from a symbolin order to properly read the symbol. Where λ is the laser wavelength,where R₀ is the starting radius of the laser beam at the systemaperture. More preferably,$\frac{\alpha}{5 \cdot R_{0}} < \beta < \frac{\alpha}{R_{0}}$

The present invention also teaches the construction of an optical systemhaving at least one optical element for partially collimating the laserbeam, providing optical power to reduce beam divergence and forproducing a wave front as described in Equation 24 below. In preferredembodiments, the optical system includes a plano convex lens forpartially collimating the laser beam and an optical element having asubstantially flat first surface perpendicular to the optical axis ofthe system and a second surface defined by a figure of rotation formedby rotating a line about the optical axis at an acute angle to define anoptical element which causes a phase tilt in the beam inward toward theoptical axis. The beam so produced has a transverse field distributionthat can be expressed as a series containing the Bessel functions.Essentially only the axial peak of beam is employed for scanning.

The present invention also includes a method of producing a laser beamfor scanning an optical code. A diverging laser beam is provided by asemiconductor laser. The divergence of the laser beam is reduced by, forexample, a lens to a predetermined non-zero divergence. A generallyconical wave front deformation is imposed on the laser beam, for exampleby a lens with a conical surface. These steps can be performed in anyorder or simultaneously. Such a diverging axicon laser beam is producedfor scanning optical code. The beam may be used in a method of scanninga symbol. According to this method, a beam of laser light is generated.The transverse intensity pattern of the beam shows a

It has been proposed to create a laser scan beam by directing acollimated beam of laser light onto a linear axicon optical element, forexample, a conical lens, to produce a beam of light which exhibits aconsistent spot size over a substantial distance along the axis of thebeam. Such an optical system is disclosed in U.S. Pat. No. 5,331,143 toMarom et al. and assigned to Symbol Technologies, Inc., which patent ishereby incorporated by reference.

The aforementioned axicon system produces a nearly diffraction freebeam. The use of such a beam has been proposed to maximize the focusinglimited working range of the scanning beam. Such a beam exhibitssubstantially no divergence over a relatively long distance range andthen breaks into a donut like spot pattern of intensity distribution.Such a non-diverging beam can provide two to three times the range of aconventional Gaussian beam for a particular bar-code density. However,as noted by applicants, where such a beam is designed to improveperformance in scanning a certain bar code density, the correspondingworking ranges of lower density symbols are not increased significantlyor at all, being limited by the distance where the beam breaks into adonut-like distribution (“donut distance”).

Accordingly, it is a primary object of the present invention to optimizeand maximize the working ranges of a bar code scanner for a wide varietyof bar code densities.

Some conventional scanners employ two laser optical systems: one a shortworking range system and the second a long working range system. Suchscanners have multiple operating modes, which allow for the selection ofa different optical system, spot sizes and scanning beam depending onthe distance of the target system. However, such systems are larger andmore complicated than single source scanners, and consequently tend tobe more expensive and less desirable for all these reasons.

It is another object of the present invention to provide a compact andinexpensive bar code scanner with improved working ranges and variety inthe bar code densities which can be read at such ranges.

It is known that in certain scanning applications, an elongated orelliptical spot may be used to improve the performance of the scanningsystem. See, for example, U.S. Pat. No. 5,648,649 to Bridgelall. When anelliptical spot is employed, ideally the major axis of the spot isoriented parallel to the major axes of the bars making up the targetcode and perpendicular to the direction of beam scan. Such a spot formmay reduce inaccuracies in reading the code, because, e.g. an ellipticalspot system is somewhat less susceptible to errors introduced by voidsand spreads in the symbol and speckle noise.

Conventionally, the elliptical spot is generated from a Gaussiandistribution laser beam by introducing a cylindrical optical power in adirection perpendicular to the scanning direction and/or employing anelliptical or rectangular exit pupil. Applicants have observed that suchtechniques are ineffective in producing an effective spot geometry whenan axicon laser beam is employed.

Accordingly, it is another object of the present invention to provideimproved techniques for producing an elongated spot geometry in a laserscanning system employing both Gaussian and non-Gaussian laser beams.

In a bar code scanner which relies on a precise scanning laser beamprofile and spot geometry, changes in temperature that affect theoptical properties of the beam producing system can degrade theperformance of the system such as by reducing effective working range,reducing readable code density at a particular distance and generallyreducing the accuracy of the system. Various techniques have beenproposed for reducing temperature variation (“athermalizing”) opticalsystems in bar code readers. Such systems are disclosed, for example, inU.S. Pat. No. 5,673,136 to Inoue and U.S. patent application Ser. No.09/109,018 filed Jul. 1, 1998 to Li et al. and assigned to SymbolTechnologies, Inc., which application is hereby incorporated byreference.

Typically glass optical elements are less affected by temperaturechanges than are plastic optical elements having the same nominaloptical properties. Typically, also, plastic optical elements are lessexpensive and less difficult to fabricate and replicate.

It is a further object of the present invention to provide improvedathermalized optical systems for laser scanners, both for conventionallaser scanners and scanners adapted to achieve the other objects of thepresent invention.

These and other objects and features of the present invention will beapparent from this written description and the associated drawings.

SUMMARY

The present invention relates to an improved axicon optical system forscanning optical codes. The improved beam-shaping systems of the presentinvention are capable of generating nearly diffraction-free beams toimprove the ability to scan bar codes. A laser beam produced thereby hasa central peak having a controlled divergence and may be used to producean elongated scanning spot. Aspects of the present invention also relateto a laser assembly which may be used to produce scanning beamsincluding the above-mentioned diverging beam with elongated spot. Theapparatus combines various optical functions in structures which arerelatively insensitive to temperature change.

The present invention includes an optical code scanner employing adiverging, nearly diffraction free laser beam to scan optical codesymbols. The source of the laser beam may include a laser diode and anoptical system positioned with respect to the laser diode and configuredto produce a diverging laser beam with a generally conical wave frontwhich flattens radially outwardly from an axis of propagation of thebeam. In preferred embodiments, the nearly diffraction-free beam has anelongated transverse intensity distribution which forms an elongatedspot on a reference surface perpendicular to an axis of propagation ofthe beam at a minimum working distance from the scanner. The centralpeak in the transverse intensity does not split into rings or donutswithin the designed working range. For example, the beam produces a spotat the minimum working distance of the scanner which has a dimension d₀in the direction of scanning which is less than 13 mils and a dimensiond, greater than 160 mils at the maximum working distance of the scanner.For example, embodiments of the scanner are capable of using the laserbeam to read 7.5 mil code at a minimum working distance of less than 9inches and to read 100 mil code at a maximum working distance greaterthan 520 inches.

The present invention also includes a laser beam for use in scanning barcodes. The inventive beam has a wave front which deviates from areference plane perpendicular to its axis of propagation, the deviationbeing characterized by a phase whose dependence is given by

W(r)=αr−βr ²

where W(r) is the deviation; where r is radial distance from the axis ofpropagation; where α is selected to produce a spot at the minimumscanning distance with a diameter d₀ sufficient to permit the reading ofthe highest density bar code to be scanned; and where β is selected toprovide optimum working ranges for bar codes of different densities. Inpreferred embodiments, α is selected between 0.2×10⁻³ and 6×10⁻³. β isselected so that$\frac{\lambda}{10 \cdot R_{0}^{2}} < \beta < \frac{2 \cdot \alpha}{R_{0}}$

central peak surrounded by a plurality of side lobes symmetricallylocated on either side of the central peak. The beam has a high degreeof energy confinement near the propagation axis when the beam travels tothe symbol. The modified beam may be moved across the symbol to scan thesymbol.

Another aspect of the present invention relates to a laser scannerapparatus for producing a laser beam with an elongated spot for scanninga symbol. The apparatus includes the use of a partially reflectivemirror, which is placed in the optical path of a circular ornon-circular diffraction-free beam. Light is partially transmitted andpartially reflected. However, the reflected portion of the beam returnsto the same direction as the transmitted beam by a second reflection onthe back of the mirror. A nearly diffraction-free beam is obtained withan elongated transverse intensity distribution by a superposition of thetwo portions of the incident beam. Advantageously, the two portions aresufficiently displaced so that optical interference of these twoportions is negligible. Centers of the beam portions may be displacedfrom one another along an axis perpendicular to a direction of scanningof the laser scanner apparatus. The result is an elongated region ofillumination on a reference plane perpendicular to the beam portions inat least part of a working range of the laser scanner apparatus.Advantageously, the beam portions are sufficiently displaced so thatthey have minimum overlap. Preferably, the optical system used toproduce the elongation is a single period diffraction grating with acosine-shaped profile.

In an alternative embodiment, the optical system includes an opticalplate having two planar refractive surfaces which are non-parallel andfrom each of which a portion of the laser beam is propagated. In yetanother alternative embodiment, the optical system includes a partiallyreflective plate having two planar reflective surfaces from each ofwhich a portion of the laser beam is propagated.

The present invention also includes apparatus and techniques forproviding a system aperture to optimize the laser beam used by the codereader based on system requirements and intended use. In accordance witha preferred embodiment, a laser beam with an elliptical spot is producedby a laser diode. A laser beam is projected from an optical code readeralong a projection axis z, for scanning an optical code in a directionx, generally perpendicular to an axis of elongation of features in theoptical code. The axis of elongation of the elliptical spot is orientedperpendicular to the scanning direction x. The dimensions of a centralpeak of the beam in planes perpendicular to the projection axis increasein both the x and y directions with distance from the code reader. Thelaser beam from the laser diode is truncated by an aperture in theoptical code reader. A change in a y dimension of the aperture changesan x dimension of the central peak of the beam, thus providing anadditional design parameter or variable.

As discussed above, the laser beam may have a wave front which deviatesfrom a reference plane perpendicular to the axis of propagation z, thedeviation being characterized by the formula:

W(r)=αr−βr ²

where W(r) is the deviation; where α is selected to produce a spot atthe minimum scanning distance with a diameter d₀ sufficient to permitthe reading of the highest density bar code to be scanned; and where βis selected to provide optimum working ranges for bar codes of differentdensities. Advantageously α is between 0.2·10⁻³ and 6·10⁻³ mm⁻¹.Advantageously β is less than α/0.5 w where w is the width of theaperture in the x direction.

The foregoing apparatus can be employed in designing optical codereaders. In the design process a source of a beam of laser light isselected, preferably a laser diode with an elliptical beam spot. Anoptical system is selected for modifying the beam of laser light tocreate a beam which has a generally conical wave front and an intensityprofile in the direction x characterized by a central peak with adiameter which increases as a function of distance from the apparatus.An aperture is selected for limiting the beam and for producing adesired dimension in the x direction of the central peak. Morespecifically, an aperture dimension perpendicular to the x direction isselected to vary the dimension of the central peak in the x direction.The selections may be made on the basis of anticipated symbol modulesize, anticipated scanning range and desired depth of field.

The aperture may be characterizable by the two orthogonal dimensions, wand h, such that a change in the h dimension of the aperture changes adimension of the spot in the x direction. In a preferred embodiment theaperture is defined by a rectangular-shaped opening with the dimension wand the dimension h being in the x and y directions, respectively. Inother embodiments the aperture is defined by an elliptical-shapedopening or by a cats-eye-shaped opening, wherein the dimensions h and wrepresent the major and minor axes of these shapes, respectively.

The present disclosure also includes techniques for enhancing thetemperature stability of the laser beam source. A lens system may beemployed having a glass lens with at least one spherical surface; and amolded plastic element having a non-planar refractive surface located onan optical axis of the glass lens. The combination of the plastic lensand the glass lens provides an aspherical lens system with essentiallyall of the optical power in the glass lens and with greater temperaturestability than an optically equivalent aspherical plastic lens. Theplastic element may include at least one surface configured to producethe generally conical wave front deformations described, herein. Themolded plastic element may also provide a single period grating orprovide an aberration correction for the glass lens. Preferably,rotationally symmetric elements can be formed on one surface of theplastic element, and cylindrical elements can be formed on the othersurface.

An athermalized laser assembly may be constructed from a laser diode, alens system, and first and second members for maintaining the relativepositions of the laser diode and lens system. The first member may be atube having a first coefficient of thermal expansion and effectivelength L in the direction of an optical axis of the laser assembly. Thesecond member may be a cylindrical sleeve. The sleeve may have a second,larger coefficient of thermal expansion and an effective length L_(p) inthe direction of the optical axis of the laser assembly. The sleeve andtube may be telescoping and together confine the separation between thelaser diode and lens system to a desired spacing along the optical axisof the assembly. The lengths L and L_(p) and the thermal properties ofthe tube and sleeve are selected so that linear expansion of the sleevein one direction along the optical axis of the assembly substantiallycancels out the linear expansion of the tube in the opposite direction.

The laser assembly may be constructed with a lens system which includesa lens with at least one spherical surface. A lens element integral withthe sleeve provides a conical surface and a single period diffractiongrating with a cosine-shaped profile. The laser diode and lens systemmay be positioned with respect to one another and configured to producea diverging, nearly diffraction free beam having an elongated region ofillumination in a reference plane perpendicular to the laser beam withina working range of the laser assembly.

The foregoing is intended to summarize certain aspects of the invention.The subject matter intended to be protected is, however, defined by theclaims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial cut away view of a hand held laser scannerillustrating certain aspects of the present invention;

FIG. 2 is a diagram showing a wave front and ray structure of a priorart non-diverging axicon beam;

FIG. 3 is a diagram showing the approximate beam profile of a prior artnon-diverging axicon beam;

FIG. 4 is a diagram showing a wave front and ray structure of a laserbeam of a preferred embodiment of the present invention;

FIG. 5 is a diagram showing the approximate beam profile of a beam of apreferred embodiment of the present invention;

FIGS. 6(a) through (h) are examples of various combinations of opticalelements which may be employed to produce a diverging, nearlydiffraction free beam of a preferred embodiment of the presentinvention;

FIGS. 7(a) and 7(b) illustrate the use of optical elements to produce anelongated spot from a laser beam in accordance with the presentinvention;

FIG. 8 illustrates the combination of optical elements including acosine profile optical surface for stretching the spot produced in anoptical system in accordance with a preferred embodiment of the presentinvention;

FIGS. 9(a) and (b) illustrate the beam diameter and modulation transferfunction (MTF) of a laser scanning beam in accordance with a preferredembodiment of the present invention;

FIGS. 10 through 14 illustrate laser spot characteristics of the beam ofFIG. 9 at various working distances including intensity distribution(a), spread function (b) and MTF (c);

FIG. 15 is a cross-sectional view of a glass lens and plastic aberrationcorrector in accordance with a preferred embodiment of the presentinvention;

FIG. 16 is a cross-sectional view of a prior art lens and laser diodestructure;

FIG. 17 is a cross-sectional view of an athermalized laser assemblyusing a plastic element;

FIG. 18 is a cross-sectional view of an athermalized optical system forproducing diverging, nearly diffraction free beams including stretchedbeams in accordance with preferred embodiments of the present invention;and

FIG. 18(a) is a detail of a plastic element used in the optical systemof FIG. 18.

FIG. 19 is a pictorial view of an apparatus for producing a laser beamfor scanning optical code in accordance with a preferred embodiment ofthe present invention.

FIGS. 20, 21 and 22 illustrate characteristics of a laser beam producedby the apparatus of FIG. 19 including beam dimensions (a) and MTF forvarious code sizes (b).

TABLE OF CONTENTS

I. Laser Beams. For Versatile Code Reading Systems

FIGS. 1, and 4 to 6

II. Radially Elongated (Stretched) Laser Beams And Associated OpticalSystems

FIGS. 7 to 14

III. Plastic Aberration Corrector For Glass Lenses

FIG. 15

IV. Athermalized Laser Assembly Using Plastic Element

FIG. 17

V. Athermalized Optical System For Producing Laser Beams, IncludingElongated, Diverging Nearly Diffraction Free Beams

FIG. 18

VII. Performance Variations With Changes In Aperture

FIGS. 19 to 22.

DETAILED DESCRIPTION

The present invention provides novel and useful laser beams withprofiles and spot geometries which can be adapted to optimized workingranges and code densities according to the intended use of the laserscanner. The present invention also provides optical systems forproducing such laser beams. The systems are readily fabricated andthermal stable.

FIG. 1 is a pictorial cut away view of a hand held laser scanner device20 illustrating certain aspects of the present invention. The device isusable in reading symbols including the one dimensional bar code symbol22. It will be understood that aspects of the present invention may alsobe used in reading two dimensional optical code symbols such as PDF codeand MaxiCode.

The laser scanner device 20 given as the example here, is of a typehaving a housing 24 with a barrel portion 26 and a pistol grip handleportion 28. The present invention may also be implemented in other typesof scanners including stationary point of sale units, pen scanners, ringscanners, etc.

In the illustrated embodiment, the barrel portion 26 of the housing isformed with an exit port or window 30 through which an outgoing laserlight beam 32 passes. The beam 32 is shown impinging on the bar codesymbol 22, at a distance from the scanning device.

The laser beam 32 scans across the bar code symbol 22 along a path A—Awhich, at least under ideal conditions is perpendicular to the principleaxes of the bars and spaces making up the symbol 22. This linearscanning movement of the laser beam 32 is generated by an oscillatingoptical element such as the pivoting mirror 34 shown in the embodimentof FIG. 1. If desired, driving mechanisms may be provided to scan thebeam 32 through a two dimensional scanning pattern to permit reading oftwo dimensional bar code symbols.

The scanner device 20 includes a source of laser light such as the laserdiode 36 shown in the embodiment of FIG. 1. The laser diode may bemounted in a tube 38, which in turn is positioned in the housing. Aphoto detector (not shown) is provided within the housing to receive atleast a portion of the light reflected by the bar code 22. The photodetector may face outwardly through the window 30, or, alternatively mayface the scan mirror 34 to receive light reflected from the symbol 22.The photo detector detects light reflected from the symbol 22 andcreates an analog electrical signal. A digitizer and decoder processessignals from the photo detector in the conventional way to deriveinformation from the symbol 22.

The laser diode produces a laser beam which, in the embodiment of FIG.1, is directed onto a lens 40 and a linear axicon lens 42. Theappropriate selection of lens surfaces and component placements as wellas various alternative embodiments are discussed in detail below. Thebeam 32 has a central peak which will produce a spot of a size whichincreases with distance from the scanner—a diverging beam. The centralspot is largely unaffected by diffraction effects. These and otherimportant properties of the beam are also discussed in detail below.

In the embodiment of FIG. 1, the laser beam exits the tube 38 and isreflected off of a folding mirror 44 toward the scan mirror 34. The scanmirror is pivoted in the direction indicated by the double-headed arrowB—B to produce a desired beam scanning pattern. The system can readcodes close to the system of some maximum density and has a maximumworking distance which may be dictated by the donut distance Z_(max),discussed below.

Laser Beams For Versatile Code Reading Systems

Preferred embodiments of the present invention employ a laser beamhaving wave front deformation at a reference plane perpendicular to anoptical axis of the system at, for example, an exit pupil of focusingsystem. The deformed wave front is described by the following formula:

W(r)=α·r−β·r ²  (1)

where

β>0  (2)

and where r is the radial distance from the optical axis and α is thecone angle of the wave front cone as defined below.

To understand properties and advantages of the beam (1), consider firstpropagation of an infinite conical wave (non-diverging axicon beam) suchas described in the above-mentioned '143 patent to Marom et al. Theproperties of such a beam are shown in FIGS. 2 and 3. The wave front ofthe beam can be described by deviation from the reference plane by anequation of the form:

W₀(r)=α·r  (3)

It is known that such a wave produces a non-diverging Bessel beam. Thediameter of the beam central peak do is determined by the wave length λand wave front cone angle α:

d ₀=0.76·λ/α  (4)

Although the theoretically infinite Bessel beam does not change withdistance, its intensity is equal to zero, so that energy is conserved.

For bar code scanners, an essentially non-divergent axicon-like laserbeam that are close to a Bessel beam bounded by a Gaussian envelope hadbeen proposed in the '143 patent providing larger working ranges thanconventional Gaussian beams. Consider such a laser beam as shown inFIGS. 2 and 3 having at a starting distance (Z=0), on the axis ofpropagation C—C, radius R₀ and a conical wave front (50). Similar to thetheoretical Bessel beam, this beam propagates practically with zerodivergence. However, the beam intensity is not zero any more. The pricepaid is that the beam breaks into a donut shaped intensity distributionafter distance Z_(max0). Z_(max0) is given by the expression:

Z _(max0) =R ₀/α  (5)

The scanner has working ranges on high and low density bar codes limitedby the distance Z_(max0). Obviously, in order to increase working rangeof the beam, one could increase Z_(max0) by increasing the beam radiusR₀. A limitation here comes from the fact that the larger R₀, the lessrelative energy goes into central maximum d₀, and the lower theModulation Transfer Function (MTF) of the scanning laser beam. Inpractice, R₀ may, for example, be a physical aperture such as a circularor elliptical mask or be dictated by the size of the beam produced bythe laser and lens system.

In accordance with the present invention, a wave front deformation isperformed consistent with equation 1. Such a wave front 52 is shown inFIG. 4. The wave front 52 is generally conical and flattened at itsradial edge. One form of the wave front is a shape generated by rotatinga curved line 53 at an acute angle a about the axis of propagation D—Dof the system. The form of the line may be selected to produce a nearlyconical wave front near the axis D—D and to produce a flattening towardreference plane 54 in radially outward portions of the wave front 52closer to the edge of the aperture.

One way to produce this beam is to transform the beam of equation 3using a lens with a focal length of F=−1/(2β) in the reference planeZ=0. One can understand this transformation based on the lens function:the plane wave front is transformed into a spherical one with radiusequal to F. In other words, the lens introduces a wave front deviationequal to sag of the sphere with radius F.For r<<F:

ΔW(r)=(F ² +r ²)−F≈0.5 r ² F

If

F=1/(2β)  (6)

Then

ΔW(r)=−β·r ², and W(r)=W ₀(r)+ΔW(r)  (7)

The profile of beam W(r) can be determined by following the rule ofgeometrical optics: The intensity distribution that takes place in thebeam W₀(r) at a distance Z₀ will take place also in the beam W(r) but atdifferent distance Z with magnification M=Z/Z₀. According to the lensformula:

1/Z−1/Z ₀=1/F=−2·β  (8)

d/d ₀ =M=Z/Z ₀=1−Z/F=1+2·β·Z  (9)

Beam W₀(r) does not diverge until donut distance Z_(max0) (see FIG. 3).According to (5), (6), and (8), donut distance of the beam W(r) will bedetermined by:

Z _(max) =Z _(max0) ·F/(F+Z _(max0))=R ₀ ·F/(R ₀ +F·α)=R ₀/(α−2·β·R₀)  (10)

Within 0<Z<Z_(max), the profile of the beam of equation (1) can bedetermined using equation (9):

d=d ₀·(1+2·β·Z)  (11)

The profile of the central portion of this beam along the axis ofpropagation D—D is shown in FIG. 5. As indicated by equation d isessentially independent of aperture size. As shown in FIG. 5, the beamdiverges approximately linearly.

The profile slope is, in effect, a beam magnification caused by thelensing. The divergence of the beam of FIG. 5 is two to three times lessthan the divergence of a truncated Gaussian beam with comparable sourceand beam radius. Unlike a Gaussian beam, the diameter d of the centralpeak is not appreciably affected by the system aperture R₀ or thediffraction caused thereby. Although the beam profile may contain sidelobes, the sharpness and diameter d of the central peak may becontrolled through the working range of the system. Diffraction does notappreciably affect the spot with distance from the scanner and thecentral peak is largely determined by geometric effects rather thandiffractive effects.

Equations (10) and (11) show advantages of the proposed beam of Equation(1) over the known non-diverging axicon beam of Equation (3):

(a) The donut distance is increased or the donut effect is eliminatedcompletely, as indicated by the following expressions:

Z _(max) =R ₀/(α−R ₀·2β)>Z _(max0), if 0<β<α/(2R ₀)  (12)

Z _(max)→∞ if β=α/(2R ₀)  (13)

The increased donut distance Z_(max) of the beam of equation (1) permitsthe increase of working ranges simultaneously for high and low-densitybar codes. Note that the beam does not break into donut at all ifβ>α/(2R₀) because Z_(max) becomes negative.

(b) The central peak diameter d of the laser beam of equation (1)changes approximately linearly with distance Z from the reference plane(Z=0) (see FIG. 5), in accordance with the relation

d(Z)=d ₀·(1+2·β·Z)  (14)

where d₀ is determined by equation (4). The linear increase of thescanner ranges with the decrease in bar code density is a useful featureof the present invention given that it is natural to scan larger barcodes from greater distances.

The following procedure can be used to choose the coefficients a and βto provide an optimum combination of working ranges for a variety of barcode densities:

(1) Choose a maximum starting beam radius R₀ (100% encircled energy) andcentral maximum diameter do based on the signal processing requirementsfor reading the highest density bar code to be read by the system.

(2) Calculate α from equation (4).

(3) Determine β based on optimum trade-off between working ranges forhigh-density and low-density bar codes. A preferred upper limit for${{\beta \quad {is}} < \frac{2\alpha}{R_{0}}},$

more preferably $\beta < {\frac{\alpha}{R_{0}}.}$

The following Table I illustrates difference between working ranges of anon-diverging axicon beam (β=0) and two diverging beams of the presentinvention. In all cases, α=1.4·10⁻³, R₀=1.75 mm, and optimum laser barcode signal processing is assumed using a first derivative signalprocessor.

TABLE I Bar-code Working ranges in inches module, mil β = 0 β = 0.4 .10⁻³ mm⁻¹ β = 0.6 . 10⁻³ mm⁻¹ 4 — — — 5 11 — — 7.5 30 30 15 10 44 58 3515 49 94 80 20 56 120 110 40 48 200 240 55 56 265 340 70 60 320 435 10060 420 627

After having α and β determined, known optical design components may beused to generate wave front (1) based upon desired location of exitpupil, axicon vertex position relative to the exit pupil, and otheroptomechanical constraints. Some of the possible optical systems forproducing the beam of FIGS. 4 and 5 are shown in FIG. 6.

FIGS. 6(a) through 6(h) are ray traces of marginal and axial rays invarious optical systems for producing the diverging nearly diffractionfree beams invention. In each Figure in which they appear, element 100is a laser diode, element 102 is a plano-convex negative lens andelement 104 is a linear axicon lens (i.e. lens with a plano surface anda conical surface of revolution made by rotating a straight line at anangle θ=α/(n−1) about the optical axis E—E, where n is the index ofrefraction of the material out of which the linear axicon lens is made.

The optical system of FIG. 6(a) employs a spherical collimation lens102, a linear axicon lens and a plano-concave positive spherical lens106, all located on the optical axis EE. Advantageously, the laser diode100 and collimation lens 102 may be part of a laser diode package. Thelaser beam from the collimation lens next impinges on the linear axiconlens where its wave front is deformed from a planar shape (coplanar withthe piano surface of the linear axicon lens 104) to a conical shape asdescribed above. The beam from the linear axicon lens is furtherdeformed by the lens 106 and defocused to form the diverging axicon beamof a preferred embodiment of the present invention.

In the optical system of FIG. 6(b) the collimation lens and linearaxicon of FIG. 6(a) have been merged into a single optical element 108which performs the functions of elements 102 and 104 of FIG. 6(a). Thesurface of the element 108 is a composite of the surface heights ofnon-planar surfaces of elements 102 and 104 of FIG. 6(a).

In the optical system of FIG. 6(c) the lens 102 and distance d from theeffective point source origin of the beam from the laser diode 100 areselected to partially collimate and focus the laser beam to produce awave front with a deformation of −βr² from a reference plane parallel tothe plane back surface of lens 102. In other words, the divergence ofthe beam 120 from the assumed point source is reduced to a lesser,non-zero divergence at 122. R₀ denotes the system aperture.

In the optical system of FIG. 6(d), the linear axicon lens has beenreversed from the orientation shown in FIG. 6(c). However, the opticalsystem functions in essentially the same way as that of FIG. 6(c), toproduce a diverging beam of a preferred embodiment of the presentinvention.

In the optical systems of FIGS. 6(e) and (f) the positions of the linearaxicon lens 104 and the lens 102 have been reversed over those shown inthe preceding Figures. Also, the physical cone angle of the conicalsurface of the lens 104 is somewhat larger than that of linear axiconsurfaces of FIGS. 6(a)-(d), in order to produce a diverging beam similarto that produced by the optical systems of those preceding Figures. Theresultant system of FIGS. 6(e) and 6(f) have a reduced depth, x, whichmay be advantageous in applications where compactness of the opticalassembly is important to the overall design.

In the optical systems of FIG. 6(g), the lens surface and linear axiconsurface have been merged to produce surface 110 of optical element 112.The system of FIG. 6(h) employs a lens 114 with linear axicon backsurface 116 and a negative power spherical surface 118. In both systems,the component separation, negative power and physical cone axis of theaxicon are selected to produce a diverging axicon beam in accordancewith equation (1).

From the foregoing, it will be apparent that there are many equivalentoptical structures for producing the laser beam of the presentinvention. It will be understood by persons skilled in the opticaldesign arts that such implementations may include mirrors, diffractiveelements, gratings, or volume holographic bodies.

II. Radially Elongated (Stretched) Laser Beams And Associated OpticalSystems

As noted above, the advantages of using an elliptical spot in laser barcode scanning is well-known. Frequently this spot shape is produced witha cylindrical lens. However, applicants have observed that this commonmechanism for producing ellipticity in Gaussian beams is not suitablefor use with axicon beams. Instead of the desired elliptical beam in thenear field and circular beam in the far field, the result is a roundspot in the near field and multiple spots at far distances. Inaccordance with the present invention, instead of using cylindricalpower, the laser beam may be split in vertical direction. Beam splittingcan be achieved by using a diffraction grating or various refractive orreflective surfaces. These embodiments will now be described.

A first approach for obtaining an expanded beam width in a directionorthogonal to the scanning, is to generate replicas or portions of thebeam, via a beam-splitter, tilted plate, etc., (see FIGS. 7(a) and7(b)). This achieves an expanded beam front without significantlyaffecting other properties of the original beam. Care needs to be takento ensure that the interference between the beams at their fringeregions when they do partially overlap, does not adversely affect theperformance of the scanner.

One way of stretching a beam is to use a mirror to split the incidentbeam and overlap the beam portions as shown in FIGS. 7(a) and 7(b). Asshown in FIG. 7(a), a mirror 200 which may be the scan mirror has a beamsplitting surface 202 and reflective surface 204. An incoming beam 206will be split into at least two beams: 208 and 210. Surfaces 202 and 204are tilted at a small angle θ from parallel. Therefore, beams 208 and210 will not be parallel. The small angle between beams 208 and 210provides the elongated or stretched laser spot.

Another method of generating an elongated beam is illustrated by theembodiment of FIG. 7(b). A partially reflective mirror 220 is placed atan angle in the optical path of a beam 222, particularly the divergingnearly diffraction free beam previously discussed. Light is partiallytransmitted and partially reflected as indicated by the rays at 224 atthe optical interface 226. For example, the surface at 226 may be 50%silvered. Light reflected at the interface 226 may be re-reflected atinterface 228, which may be an essentially 100% reflective surface. Theresult may be two partially overlapping beam portions 230 and 232. Theseparation distance S between the centers of the beam portions,advantageously, remains constant through the working range of thescanner. Alternative beam portions 234 and 236 are also shown which areseparated so that there is no overlap of their central maxima, hence noappreciable interference effects. In this embodiment, the dimension ofthe spot, perpendicular to the scanning direction is effectivelydoubled.

A preferred method of producing an elongated or stretched beam makes theuse of what may be thought of a diffraction grating. The approach isbased on a device consisting of a course (low spatial frequency) gratingpositioned over the beam aperture. This could be achieved with anamplitude grating (which is partially absorbing, thus reducing the powerthroughput) or a phase grating (which is non-absorbing, and can providegreater beam stretching in view of the many diffraction orders that itcontains). In the following embodiments, systems are described using asimple uniform aperture. Nevertheless, the results are expected to besimilar for arbitrarily shaped apertures.

In a preferred embodiment, beam elongation is achieved using a verysmall diffracting angle which corresponds to only one period of agrating defined by the equation: $\begin{matrix}{{G(y)} = {G_{0}{\cos \left( {2\pi \frac{y}{A}} \right)}}} & (15)\end{matrix}$

where G is the grating phase, y is the lateral coordinate perpendicularto the scanning direction, A is the vertical aperture, and G₀ is theamplitude of the grating phase.

Equation (15) describes a cosine surface. Its optical power over theaperture is equal 0. As will be seen in the example presented below,this technique generates a smooth elongated spot in the near field. Thetechnique is useful in application to axicon beams lacking inherentellipticity. It is also useful in adding ellipticity to otherconventional beams.

A system for producing an elongated, diverging beam is illustrated inFIG. 8. The system consists of a laser beam source 250, positive lens252, linear axicon lens 254 and lens 256 having a surface 257 generatedby a line perpendicular to the optical axis F—F which tracks the cosinecurve of equation (15). It will be observed that the components andtheir arrangement in FIG. 8 are similar to FIG. 6(c) discussed above,but with the addition of the cosine surface lens 256. The resultant beampropagates in a direction along optical axis F—F. The spot 258 isstretched or elongated in the y direction.

The following is a theoretical evaluation of the use of a grating toachieve beam elongation. Let a flat aperture (1−D) be defined as “rect(x/a)” which is equal to 1 for |x|<a/2 and 0 elsewhere. Then in the “farfield” one gets $\begin{matrix}\left. {{rect}\left( \frac{x}{a} \right)}\Leftrightarrow{a\quad \sin \quad {c\left( {af}_{x} \right)}} \right. & (16)\end{matrix}$

where “→” denotes the Fourier transform, $\begin{matrix}{{\sin \quad {c\left( {af}_{x} \right)}} = \frac{\sin \left( {\pi \quad {af}_{x}} \right)}{\pi \quad {af}_{x}}} & (17)\end{matrix}$

and ƒ_(x) is the spacial frequency along the x-axis in the plane of theFourier spectrum. Multiplying this function with an amplitude coarsegrating (of period “a”): $\begin{matrix}{{t(x)} = {\frac{1}{2}\left\lbrack {1 + {\cos \left( {2\pi \frac{x}{a}} \right)}} \right\rbrack}} & (18)\end{matrix}$

The Fourier transform of the product of the functions given by Eqs. (16)and (18) leads to the following expression $\begin{matrix}\left. {{{rect}\left( \frac{x}{a} \right)}{t(x)}}\Leftrightarrow{\frac{a}{2}\left\{ {{\sin \quad {c\left( {af}_{x} \right)}} + {\frac{1}{2}\left\lbrack {{\sin \quad {c\left( {{af}_{x} + 1} \right)}} + {\sin \quad {c\left( {{af}_{x} - 1} \right)}}} \right\rbrack}} \right\}} \right. & (19)\end{matrix}$

for the far field of diffraction. Furthermore, if a phase-only gratingis used, the transmittance can be expressed as $\begin{matrix}{{t(x)} = {\exp \left\lbrack {{\gamma}\quad {\cos \left( {2\pi \frac{x}{a}} \right)}} \right\rbrack}} & (20)\end{matrix}$

Incidentally such a grating may be achieved either by diffractive means(DOE) or refractive ones. The relation between γ and the physical heighth of the grating variations is $\begin{matrix}{\gamma = {\frac{2\pi}{\lambda}\left( {n - 1} \right)h}} & (21)\end{matrix}$

where λ is the light wavelength and n the index of refraction of thematerial, assumed to be surrounded by air (n=1).

Since the phase grating has also a periodicity “α”, it can be expandedin a Fourier series, thus leading to $\begin{matrix}{{\exp \left\lbrack {{\gamma cos}\left( {2\pi \frac{x}{a}} \right)} \right\rbrack} = {\sum\limits_{n}{{J_{n}(\gamma)}{\exp \left( {{2n\quad {\pi }} - \frac{x}{a}} \right)}}}} & (22)\end{matrix}$

where J_(n) is the Bessel function of order n. Thus, when a phase maskmultiplies the input, one gets in the far field: $\begin{matrix}{\left. {{{rect}\left( \frac{x}{a} \right)}{\exp \left\lbrack {{\gamma cos}\left( {2\pi \frac{x}{a}} \right)} \right\rbrack}}\Leftrightarrow{a\quad \sin \quad {c\left( {af}_{x} \right)}*{\sum\limits_{n}{{J_{n}(\gamma)}{\delta \left( {f_{x} - \frac{n}{a}} \right)}}}} \right. = {a{\sum\limits_{n}{{J_{n}(\gamma)}\sin \quad {c\left( {{af}_{x} - n} \right)}}}}} & (23)\end{matrix}$

The result thus consists of several replicas of the original patternpropagating along the different diffraction orders and weightedaccording to the depth of the grating corrugation. As noted above, it isproposed to use a defocused beam with wave front at the exit pupil:

W(r)=α·r−β·r ²,  (1)

where r is the radial coordinate, α is the cone coefficient, and β isthe defocus coefficient. To achieve beam stretching, the laser beam iselongated in the direction perpendicular to the scanning using aone-period diffraction grating which, in generalization, can beexpressed in the form: $\begin{matrix}{{G(y)} = {\sum\limits_{m = 1}^{M}\quad {g_{m}{\cos \left( {2m\quad \pi \frac{y}{A}} \right)}}}} & \text{(15a)}\end{matrix}$

where G(y) is the phase delay suffered by the wave at coordinate y inpassing through the element, A is the linear dimension of the aperturein the y-direction and M is an integer. In the preferred case of M=1,Eq. (15a) reduces to the form $\begin{matrix}{{G(y)} = {g_{1}{\cos \left( {2\pi \frac{y}{A}} \right)}}} & \text{(15b)}\end{matrix}$

where y is the vertical coordinate, A is the vertical aperturedimensions, and w₁ is the amplitude of wave front deformation.

Combining equations (1) and (15), the optimum diverging elongated beamcan be obtained when using the following wave front deformation at thesystem exit pupil: $\begin{matrix}{{W\left( {x,y} \right)} = \quad {{a\sqrt{x^{2} + {ɛ\quad y^{2}}}} - {\beta \left( {x^{2} + y_{2}} \right)} + {\sum\limits_{m = 1}^{M}\quad {w_{m}{\cos \left( {2m\quad \pi \frac{y}{A}} \right)}}} + {\gamma \quad y^{2}}}} & (24)\end{matrix}$

The following values are typical:

α=(0.2−6)×10⁻³;ε=1;γ=0;

and $\frac{\lambda}{10R_{0}^{2}} < \beta < {2{\frac{\alpha}{R_{0}}.}}$

The following example illustrates the properties of a beam generated inaccordance with equation (24) optimized for variety of bar-codedensities ranging from 7.1 mil to 100 mil and having spot ellipticity upto 110 inches from exit pupil. The beam is predicted to provide thefollowing working ranges (measured from exit pupil):

Bar-code module, mil Working range, inches 7.5 From 8.5 to 25 10 From8.5 to 42 15 to 81  20 to 112 40 to 240 55 to 330 70 to 420 100 to 600Beam parameters (x is scanning direction, y is perpendicular one): (a)Laser wavelength: 635 nm divergence angles: 8° × 28° (x x y) (b) Lensfocal length: 9.2 mm aperture: 3 × 2.5 mm (x x y) (c) Wave frontdeformation parameters: cone angle coefficient α = 1.4 . 10⁻³ defocuscoefficient β = 0.6 . 10⁻³ mm⁻¹ amplitude y-axis W₀ = 380 nm

FIG. 9(a) shows the Focusing Profile of the laser beam and FIG. 9(b)shows MTF of the system. The focusing profile illustrates how beamdiameter (in microns) diverges as distance (in inches) from the scannerincreases. Beam diameters in the horizontal scanning) and verticalplanes are calculated at 50% intensity level. FIG. 9(b) illustrates howthe MTF of the system varies with code density and distance.

FIGS. 10-14 show laser spot characteristics at various workingdistances: 8.5, 25, 81, 112 and 600 inches, respectively. In particular,FIG. 10 illustrates laser spot characteristics at a minimum workingdistance of 8.5 inches. The size and elongation of the spot isillustrated by the intensity distribution of FIG. 10(a). The intensityprofile of the spot is shown along the x and y axis in the graph of theLine Spread Functions (LSF) in FIG. 10(b). It will be observed that theeffective spot dimension in the y-direction is greater than in the xdirection i.e. the spot is elongated. The horizontal MTF at 8.5 inchesis presented as a function of bar code module size in mils in FIG.10(c).

Similar data is presented for the same beam at greater working distancesin FIGS. 11-14 which represent maximum working distances for 7.5 mil, 15mil, 20 mil and 100 mil bar code modules, respectively. It will beobserved that the y axis elongation or stretching of the spot issubstantially greater at short working distances and graduallydiminishes as working distance increases.

III. Plastic Aberration Corrector For Glass Lenses

The most economical way for mass producing aspherical lenses is plasticmolding. However, the strong temperature dependence of the refractiveindex of plastic causes undesirable lens focal plane shifting withtemperature. Therefore, plastic lenses are typically not used inapplications where stable focal distance is required over a widetemperature range.

There are several known mass production methods to maketemperature-stable aspherical lenses for laser beam transformations(collimating, focusing, etc.):

(a) spherical glass lens with attached plastic aspherical replica;

(b) aspherical glass molded lens;

(c) plastic athermalized aspherical lens using combination ofdiffractive and refractive optical power.

The first two methods use specialized optical technology that may leadto an increased part price. The third type of lens system suffers fromenergy losses at the diffractive surface.

It is an object of the present invention is to provide a high-quality,cost-effective, mass producible and temperature-stable aspherical lenssystem. The technique is advantageously used in both conventional laserbar code scanners and in scanners using the elongated, diverging laserbeams disclosed herein.

In accordance with the present invention, a spherical glass lens is usedwith a separate plastic phase plate for aberration correction. In thesimplest embodiment, all optical power is provided by a stable glasscomponent. The phase plate has little or no optical power and providescorrection of the spherical aberrations of the lens. Optical pathdifference (OPD) introduced by a refractive component into thetransmitted wave front is determined by the component refractive index nand thickness t as function of radius r:

W(r)=(t(r)−t(0))·(1−n)  (25)

Change of OPD due to temperature dependence of refraction index is givenby the expression $\begin{matrix}{{\Delta \quad W} = {{\Delta \quad T*\frac{\partial W}{\partial n}\frac{\partial n}{\partial T}} = {\Delta \quad T*{\frac{\partial n}{\partial T} \cdot \frac{W}{n - 1}}}}} & (26)\end{matrix}$

The smaller the nominal OPD (W) introduced by the refractive componentis, the less the wave front distortion ΔW caused by temperaturedependence of refractive index ∂n/_(∂T) is. This is why a plasticcomponent having high coefficient ∂n/_(∂T) can be a stable aberrationcorrector although not stable as a lens.

FIG. 15 shows a glass lens 300 with a spherical surface 301 used incombination with a separate plastic phase plate 302 for aberrationcorrection. The surface 304 of the plate 302 may include a sphericalaberration correction component given by the expression:

a·r ⁴ +b·r ⁶+ . . .

where a is a constant and r is the radial distance from the optical axis(27) G—G.

IV. Athermalized Laser Assembly Using Plastic Element

The focusing stability of a conventional laser assembly of a bar-codescanner is affected mainly by thermal expansion of the housing. This isespecially true when using, in high volume production, cast metalcomponents (for example, cast zinc) having a large thermal expansioncoefficient. Thermal focusing shift then becomes a limiting factor ofhigh-end bar-code scanner performance.

The purpose of the invention is to provide a cost effective, thermallystable focusing solution for high-volume, high-performance, bar-codescanners and particularly for bar-code scanners using the elongated,diverging beam of a preferred embodiment of the present invention.

FIG. 16 shows a conventional laser assembly for a bar-code scanner. Theassembly includes laser diode 320, glass lens 322, metal lens holder324, metal laser holder tube 326, and spring 328. The lens may beattached to the lens holder 324 by glue applied to an annular rim 330.By adjusting the position of the lens holder 324 relative to the holdertube 326 and attached laser 320, the lens is set at the designeddistance from the laser. The distance between the laser and the lensfocal plane changes with temperature due to thermal expansions of theglass and metal and also due to change of the refractive index of theglass. When using cast metal holders 324 and 326, their thermalexpansion/shrinkage plays the major role in focusing change. The thermalchange of the distance between laser and the lens flange can bedetermined as:

 Δx=ΔT·Cm·L  (28)

where Cm is the thermal expansion coefficient of the metal holders, andL is the distance between laser and lens flanges.

FIG. 17 shows a thermally-compensated laser assembly of a preferredembodiment of the present invention. It consists of laser diode 340,glass lens 342, plastic lens holder 344, laser holder tube 346, metallocking ring 348, and spring 350. The lens 342 may be glued to theannular rim 352 of the plastic lens holder 344. When temperatureincreases, the laser holder 346 will expand, moving lens 342 away fromlaser 340. A holder 344 is preferably made from a material having athermal expansion coefficient relatively higher than the laser holdertube 346. For example, the holder 344 may be made from a higher thermalexpansion coefficient plastic, while the tube 346 is made of arelatively lower thermal expansion coefficient metal. The lens holder344, having a length Lp selected as described below, tends to move thelens back into its original position relative to the laser, and therebycancel out the effects of thermal expansion.

The length of the plastic holder Lp can be chosen based on the thermalexpansion coefficients of laser holder tube 346, lens holder 344, andglass lens 342. Change of the glass refraction index also can be takeninto consideration.

To illustrate a choice of the plastic holder length Lp, let us assumethat the lens does not expand, and its refraction index does not changewith temperature. In this case, the amount of defocus can be calculatedas

Δx=ΔT·(Cm·Lm−Cp·Lp)  (29)

Lm=L+Lp  (30)

where Cm and Cp are the thermal expansion coefficients of metal andplastic respectively, L is the separation between the laser diode andthe lens, and Lm and Lp are the flange length of metal laser holder tubeand plastic lens holder, respectively. From equations (29) and (30), onecan obtain the length Lp that provides the thermal stability forfocusing: $\begin{matrix}{{\Delta \quad x} = {{0\quad {if}\quad {Lp}} = {L \cdot \frac{Cm}{{Cp} - {Cm}}}}} & (31)\end{matrix}$

For example, if L=10 mm, Cm=27·10⁻⁶ (cast zinc), and Cp=65·10⁻⁶(acrylic), then according to equation (31) Lp should be selected to be7.1 mm.

In a particular optomechanical design, the plastic lens holder can bereplaced by a separate plastic part. Also, length Lp can be chosen forfull thermal compensation including thermal changes of the glass lensmaterial.

V. Athermalized Optical System For Producing Laser Beams, IncludingElongated, Nearly Diffraction Free Laser Beams

Various of the techniques previously described can be combined toprovide an athermalized optical system for producing laser beams,including elongated diverging laser beams of the preferred embodimentsdescribed in Sections I and II above. A molded plastic element orcomponent may be used to implement some of the techniques.

FIG. 18 illustrates an optical system 400 of a preferred embodiment ofthe present invention. The plastic element 402 is shown in the systemand in the detail FIG. 18(a). The system includes a laser diode 404mounted in a metal tube 406 on the optical axis 0—0. A glass lens 408 isheld in position by a compression spring 410 and the plastic element402. Advantageously, glue may be applied at shoulder regions 412 of theplastic element to maintain the glass lens in position. A metal nut 414may be employed to secure the plastic element 402 in place.

As shown in detail of FIG. 18(a) the molded plastic element 402 isformed with shoulder surfaces 416 to retain the glass lens. Surfaces 418and 420 are optical surfaces whose design and function is describedbelow. The aperture 422 of the optical system is defined by the surface418 and bounded by annular edges 424 of the plastic element.

In a more preferred embodiment, a linear axicon is formed in surface418. The glass lens is a piano spherical lens to provide the necessaryoptical power. In the most preferred embodiment, the glass lens andseparation distance between the laser diode and the lens are selected toprovide the β term portion of the wave front deformation set forth inequation (1). Thereby, a diverging, nearly diffraction free laser beamis formed. Surfaces 418 and 420 may be used to introduce an optical pathdifference (OPD) between axial ray and a ray with coordinates (x,y)determined by the formula:

OPD=C ₁·(x ² +y ²)² +C ₂ ·{square root over (x²+C₃·y²)} +C ₄·cos (C ₅·y)+C ₆ ·x ² +C ₇ ·y ²  (32)

where x is the axis in the scanning direction, and y is perpendicular tox. The coefficient C₁ can be chosen to provide compensation for theglass lens spherical aberration as described in Section III above. Anelement exhibiting such behavior can be generated as a diffractiveoptical element (DOE). DOE's are conventional optical element and usesof them are described in the '143 patent mentioned above.

Coefficient C₂ can be chosen to generate a laser beam with optimizedworking ranges and module size ranges as discussed in Section I, above.More specifically, the coefficient C₂ may be chosen to equal α where αis the cone angle of the wave front as presented in equation 1.

Coefficients C₃, C₄ and C₅ can be chosen to provide desired laser beamstretching perpendicular to the scan lines. The coefficients C₄ and C₅corresponds to the terms W₀ and $\frac{2\pi}{A}$

in equation (15) and provide the cosine grating described in Section IIabove. In a preferred embodiment C₃ is set equal to one. However, C₃ maybe adjusted to add some ellipticity to the spot.

Coefficients C₆ and C₇ can be chosen to provide correction of laserastigmatism, additional beam stretching, and to minimize residualoptical power of the plastic component.

Length L_(p) of the plastic part can be chosen to provide thermo-stablefocusing as described in Section IV above.

VI. Performance Variations with Changes in Aperture

FIG. 19 is a pictorial view of an apparatus for producing a laser beamfor scanning optical code in accordance with a preferred embodiment ofthe present invention. The apparatus is a basis for examples of beamsand system performance obtained by using various apertures. FIG. 19 andthe description herein is based on the geometry of three, mutuallyperpendicular (orthogonal) directions, x, y & z. The directions, x, yand z, are used to describe the structure of the laser beam source 500,the orientation of a bar code symbol 502 located an arbitrary distance,D, from the beam source and characteristics of the laser beam 504.

In preferred embodiments, the laser beam 504 is projected in thedirection z. In operation, the laser beam may be scanned back and forthacross the symbol 502 in the direction x. This may be accomplished in aconventional manner, for example, by reflecting the laser beam off amoving mirror (not shown). However, for ease in understanding, FIG. 19is drawn so that the projection axis of the laser 506, the optical axisof optical system 508 and the projection axis of the beam 504 are allco-linear and oriented in the z direction, to demonstrate the relativeorientation of the code symbol, laser beam and laser beam aperture. Inreal systems the optical axis of the system may be folded and/ordeflected. It will be understood that the directions x and y will bereoriented in a corresponding manner so that they remain mutuallyperpendicular to the axis z.

The beam 504 may scan across the symbol 502 in a direction x generallyperpendicular to an axis of elongation (i.e. the y direction) offeatures of the optical code such as the bars 511 which make up thesymbol 502. Advantageously, the laser 506 is a laser diode whose outputbeam forms an elliptical spot. The laser diode, and optics are arrangedso that the axis of elongation of the elliptical spot is perpendicularto the scanning direction x, i.e., parallel to the bars 511.

The optical system 508 may be one of the structures shown and describedin connection with FIG. 6 and includes an aperture. The aperture may bedefined in a conventional manner, for example by an aperture plate, byperipheral edges of one or more lens elements in the optical system suchas the edge of the axicon, etc. The aperture preferably has a horizontal(x direction) dimension, w, and a vertical (y direction) dimension, h,which dimensions are selected in accordance with performance criteriasuch as anticipated symbol module size, anticipated scanning range,desired depth of field, etc., as exemplified below. For example, theaperture may be rectangular-shaped, elliptical-shaped orcats-eye-shaped.

In the particular embodiment of FIG. 19, the optical system 508 includesa spherical collimation lens 510, a linear axicon lens 512 and anaperture plate 514. The lens 510 and distance from the effective sourceof the beam from the laser diode 506 are selected to partially collumateand focus the laser beam to produce a wave front with a deformation of−βr² from a reference plane parallel to the plane back surface of lens510, as described in Section I above. Advantageously, the upper limit ofβ is inversely proportional to the aperture dimension, w, and inaccordance with the formula: β<α/0.5 w. The beam 504 has a non-zerodivergence as indicated by the gradially expanding beam size representedby the sloping lines which diverge moving from left to right in FIG. 19.

The aperture plate 514 has a rectangular opening with a width dimension,w, (x direction) and a height dimension, h, (y direction). A change inthe h dimension changes the beam spot in the x direction as describedbelow. The aperture plate 514 is located approximately in the plane ofthe conical portion of the axicon lens 512 (the conical portion of thelens 512 typically has a dimension from vertex to base of on the orderof 0.7 microns).

FIGS. 20, 21 and 22 illustrate characteristics of a laser beam producedby the apparatus of FIG. 19 assuming three different aperture heights h.More specifically, FIGS. 20, 21 and 22 show beam dimensions (a) and MTFfor various code sizes (b), for each of these aperture heights, h of arectangular aperture. For purposes of these figures aperture width, w,is held constant at 3 mm.

In the embodiment of FIG. 20 the height dimension of the laser spot atthe axicon lens 512 has been used to control the working ranges of thebar-code scanner based on signal processing capabilities. FIG. 20employs an aperture height dimension of 2.5 mm. This dimension wasoptimized for an MTF of 0.1 which is assumed to be sufficient forprocessing by a digitizer associated with the system. For different MTFlevels, an optimum vertical aperture may be different.

FIG. 20(a) illustrates the calculated x direction dimension 600 and ydirection dimension 602 of the beam as it propagates from the system.The dimensions represent a 50% intensity level of the central peak ofthe beam. The x direction dimension 600 gradually increases withdistance from the system (i.e. the beam diverges) without breaking intoa donut intensity distribution, even at a distance of 700 inches fromthe scanner.

FIG. 20(b) illustrates the MTF for eight different code symbol modulesizes from 7.5 mil to 100.0 mil bar code. As shown in the Figure thereis a useful working range (MTF≧0.1) for all eight module sizes.

The effect of changing the dimension h of the aperture, while holdingother system parameters constant, is illustrated in FIGS. 21 and 22. InFIG. 21, h is reduced to 1.5 mm. In FIG. 22, h is increased to 3.5 mm.The effect on the spot size of the system is illustrated in FIGS. 21(a)and 22(b). It will be observed that the change in the dimension heffects the spot size in both the x and y directions.

The effect of changing the h dimension of the aperture on the variousworking ranges of the system is illustrated by comparing FIGS. 20(b),21(b) and 22(b). The increase and decrease of the aperture dimension hdecreases working ranges, indicating that the system design of FIG. 20is preferred. Thus, the aperture dimensions may be selected on the basisof anticipated target symbol module size, anticipated scanning range anddesired depth of field for each. Thus, there is provided an additionaldesign variable, h, to provide the ability to better match the signalprocessing capabilities and performance requirements of the system.

The use of a laser diode with an elliptical beam spot provides greaterfreedom to adjust the dimension h of the aperture. It will be understoodthat beam truncation in the vertical direction is readily tolerated topermit optimization of the beam width for the anticipated symbol modulesizes, scanning range, depths of field and digitizer capabilities.

While aspects of the present invention have been described withreference to preferred embodiments and examples, the invention to beprotected is defined by the literal language of the following claims andequivalents thereof.

We claim:
 1. A laser beam projected from an optical code reader along apropagation axis z, for scanning an optical code in a direction x,generally perpendicular to an axis of elongation of features in theoptical code, the features extending along a direction y generallyperpendicular to the x direction, wherein dimensions of a central peakof the beam in planes perpendicular to the propagation axis increase inboth the x and y directions with distance from the code reader, whereinthe laser beam is truncated by an aperture in the optical code reader,wherein a change in a y dimension of the aperture changes an x dimensionof the central peak of the beam, and wherein the laser beam has a wavefront having a deviation W(r) relative to a reference plane generallyperpendicular to the propagation axis, the deviation being characterizedby: W(r)=αr−βr ² wherein r is a radial distance from the propagationaxis, wherein α is selected to produce a spot of the beam at a minimumscanning distance with a diameter d₀ sufficient to permit reading of thecode at its highest density, and wherein β is selected to provideoptimum working ranges for codes of different densities.
 2. The laserbeam of claim 1, wherein α is between 0.2·10⁻³ and 6·10⁻³ mm⁻¹.
 3. Thelaser beam of claim 1, wherein β<α/0.5 w where w is a width of theaperture in the x direction.
 4. A method of shaping a laser beam forscanning an optical code in a scanning direction x, comprising the stepsof: a) projecting the laser beam along a propagation axis to the code;b) selecting an optical system for modifying the laser beam to have agenerally conical wave front and an intensity profile in the x directioncharacterized by a central peak with a diameter which increases as afunction of distance from the system the wave front having a deviationW(r) relative to a reference plane generally perpendicular to thepropagation axis, the deviation being characterized by: W(r)=αr−βr ²wherein r is a radial distance from the propagation axis, wherein α isselected to produce a spot of the beam at a minimum scanning distancewith a diameter d₀ sufficient to permit reading of the code at itshighest density, and wherein β is selected to provide optimum workingranges for codes of different densities; c) selecting an aperture forlimiting the beam and for producing a desired dimension in the xdirection of the central peak.
 5. The method of claim 4, wherein anaperture dimension perpendicular to the x direction is selected to varythe dimension of the central peak in the x direction.
 6. The method ofclaim 4, wherein, in addition to the central peak, a plurality ofsecondary peaks is produced, the secondary peaks being located on eitherside of the central peak.
 7. The method of claim 4, wherein the centralpeak has a dimension which increases as a function of distance from thesystem by a factor of between 0.2·10⁻³ and 6·10⁻³ per mm of distanceprojected.
 8. The method of claim 4, wherein the selections are made onthe basis of anticipated module size of features in the code,anticipated scanning range and desired depth of field.
 9. The method ofclaim 4, wherein the laser beam produces an elliptical spot elongated ina direction perpendicular to the x direction.
 10. An apparatus forscanning an optical code, comprising: a source of a scanning laser beamfor directing the laser beam along a propagation axis toward the opticalcode to be scanned and causing the beam to move in a scan direction x;and a light detector for receiving light reflected from the opticalcode, wherein the source of the laser beam includes a laser and anoptical system positioned with respect to the laser, the optical systembeing configured to produce a generally conically-shaped wave front inthe laser beam and a central spot which increases in useable area withdistance from the scanning apparatus, wherein the source of the laserbeam has an aperture having two orthogonal dimensions, w and h, w beinga dimension in the x direction, wherein a change in the dimension of theaperture changes a dimension of the spot in the x direction, and whereinthe wave front has a deviation W(r) relative to a reference planegenerally perpendicular to the propagation axis, the deviation beingcharacterized by: W(r)=αr−βr ² wherein r is a radial distance from thepropagation axis, wherein α is selected to produce a spot of the beam ata minimum scanning distance with a diameter d₀ sufficient to permitreading of the code at its highest density, and wherein β is selected toprovide optimum working ranges for codes of different densities.
 11. Theapparatus of claim 10, wherein the laser is a laser diode which producesan elliptical beam spot elongated in a y direction perpendicular to thex direction.
 12. The apparatus of claim 11, wherein the aperture isdefined by a rectangularly-shaped opening with the dimension w and thedimension h being in the x and y directions, respectively.
 13. Theapparatus of claim 11, wherein the aperture is defined by anelliptically-shaped opening.
 14. The apparatus of claim 11, wherein theaperture is defined by a cat's-eye-shaped opening.
 15. The apparatus ofclaim 10, wherein the optical system includes at least one opticalelement for partially collimating the laser beam, and for imposing agenerally conical wave front deformation on the laser beam directedtoward the optical code.
 16. The apparatus of claim 10, wherein theoptical system includes a plano-convex lens for partially collimatingthe laser beam and an optical element having a substantially flat firstsurface perpendicular to the propagation axis, and a second surfacedefined by a figure of rotation formed by rotating a line about thepropagation axis at an acute angle with respect thereto, therebydefining the optical element which causes a phase tilt in the beaminward toward the propagation axis.
 17. The apparatus of claim 16,wherein the aperture is defined by an aperture plate locatedapproximately in a plane of the figure of rotation defining the secondsurface of the optical element.
 18. The apparatus of claim 10, whereinthe optical system includes a collimating lens, a lens providingmagnification to cause divergence of the resulting scanning laser beam,and a linear axicon lens to produce the conical wave front deformationin the laser beam.
 19. The apparatus of claim 18, wherein the apertureis defined by a rectangular aperture opening associated with the axiconlens.
 20. The apparatus of claim 10, wherein the scanning laser beam hasa transverse field distribution that is expressed as a series containingBessel functions, and wherein essentially only a central peak isemployed for scanning.
 21. A method of producing a laser beam for a barcode scanner which scans the bar code in a direction x, comprising thesteps of: a) emitting a laser beam from a laser diode along apropagation axis and having an elliptical spot elongated in a directiony perpendicular to the x direction; b) imposing a generally conical wavefront deformation on the laser beam, and wherein the wave front has adeviation W(r) relative to a reference plane generally perpendicular tothe propagation axis, the deviation being characterized by: W(r)=αr−βr ²wherein r is a radial distance from the propagation axis, wherein α isselected to produce a spot of the beam at a minimum scanning distancewith a diameter d₀ sufficient to permit reading of the code at itshighest density, and wherein β is selected to provide optimum workingranges for codes of different densities; c) selecting a dimension for anaperture in the y direction perpendicular to the x direction to obtainthe spot having a dimension in the x direction which is optimized forsignal processing of the bar code scanner; and d) limiting the laserbeam with the selected aperture.
 22. The method of claim 21, wherein thelaser beam produced is a diverging, nearly diffraction-free laser beam.